In recent years, as high performance and miniaturization of an imaging element i.e. a solid-state imaging element such as a CCD (Charged Coupled Device) image sensor or a CMOS (Complementary Metal Oxide Semiconductor) image sensor have developed, digital apparatuses such as mobile phones or personal digital assistants incorporated with an imaging device using such an imaging element have been widely spread. There is also an increasing demand for miniaturization and high performance of an imaging optical system (imaging lens) for forming an optical image of an object on a light receiving surface of the solid-state imaging element to be loaded in such an imaging device. In particular, in recent years, as the technology of increasing the pixel density in a solid-state imaging element has progressed, there is an increasing demand for high resolution in an imaging optical system. On the other hand, miniaturization of an imaging optical system has also been required as having been required in the conventional art. As an imaging optical system to be used for the above purposes, an optical system incorporated with five lens elements has been proposed, because the optical system incorporated with five lens elements provides high performance, as compared with an optical system incorporated with three lens elements or four lens elements.
Examples of the imaging optical system are disclosed in patent literature 1 and patent literature 2. The imaging lens disclosed in patent literature 1 is an imaging lens for a solid-state imaging element, and is provided with, in the order from the object side, a first lens element having a positive refractive power and convex toward the object side on the optical axis, a second lens element having a negative refractive power and concave toward the image side on the optical axis, a third lens element having a meniscus shape and convex toward the object side on the optical axis, a fourth lens element having a meniscus shape, having a positive refractive power and convex toward the image side on the optical axis, and a fifth lens element having a meniscus shape, having a negative refractive power, and concave toward the image side on the optical axis.
Further, the imaging lens disclosed in patent literature 2 is provided with, in the order from the object side, a first lens element having a positive refractive power, a second lens element having a negative refractive power, a third lens element having a positive refractive power, a fourth lens element having a positive refractive power, and a fifth lens element having a negative refractive power.
In an imaging optical system, various proposals for miniaturization have been provided, as exemplified by the imaging lenses disclosed in patent literature 1 and patent literature 2. However, there is further room for improvement on miniaturization (ultra-miniaturization). Miniaturization (ultra-miniaturization) in the specification means satisfying a requirement: L/2Y<0.8, which will be described later.
In view of the above point, for instance, patent literature 1 may fail to optimize the refractive powers of the first to third lens elements. Further, patent literature 2 may fail to optimize the refractive power of the fourth lens element. Thus, the imaging lenses disclosed in patent literature 1 and patent literature 2 may not have achieved sufficient miniaturization (ultra-miniaturization).